1. Field of the Invention
The present invention relates to a vertical cavity surface emitting laser (VCSEL), and more particularly relates to a VCSEL that involves mode control.
2. Description of the Related Art
In short-distance optical fiber communication, transmission at a rate of 10 Gb/s over a distance of about 100 m has been achieved on a practical level by means of a VCSEL, working as a laser light source, and a multimode optical fiber, working as a transmission path.
Multimode (transverse mode) laser light emitted from a VCSEL propagates in a multimode optical fiber while it couples with corresponding eigenmodes of the multimode optical fiber. Since propagation velocity varies depending on the eigenmodes, the signal waveform that is received may be deteriorated. The characteristic of the VCSEL that causes the propagation velocity of laser light to vary depending on the eigenmodes is called “mode dispersion characteristic”. When multimode optical fibers are used, the transmission performance (velocity and distance) in short-distance optical fiber communication is limited by the “mode dispersion characteristic” of the VCSEL.
In order to reduce the mode dispersion and thereby improve the transmission performance, it is necessary to reduce the difference among the propagation delays, which correspond to the respective modes of light propagating in the optical fiber. In order to reduce the above difference, it is desirable to determine the light propagation mode such that the difference between the maximum propagation delay and the minimum propagation delay is minimized. US2007/0153861 discloses a VCSEL having a protrusion, called a mirror extension, provided in the center of the upper mirror stack layer. Such a protrusion increases the reflectance ratio of the upper mirror stack layer so that the central region, provided with the protrusion, has a higher reflectance ratio than the peripheral region. As a result, low order modes having high electric field intensity in the central region oscillate more easily than high order modes that have high electric field intensity in the peripheral region.
However, it is desirable to limit the oscillation of the fundamental mode or lower modes and to promote the oscillation of higher modes in order to obtain desirable results for various oscillation wavelengths. For example, in “Advanced multimode fiber for high velocity, short reach interconnect,” Proc. of SPIE Vol. 7134, 71341L-1, 2008, FIG. 2 (see FIG. 1), Y. Sun, R. Lingle, G. Oulundsen, A. H. McCurdy, D. S. Vaidya, D. Mazzarese and T. Irujo show the relationship between eigenmodes and propagation delays (mode delays) for various oscillation wavelengths. In the case where the wavelength is 850 nm, the propagation delay for the 1st to 9th order eigenmodes decreases as the order increases and the propagation delay for eigenmodes higher than the 9th order increases as the order increases. In the case where the wavelengths are longer than 850 nm (990 nm, 1300 nm), the propagation delay monotonously increases as the order increases. Therefore, in the case where the wavelength is 850 nm, for instance, the difference between the maximum propagation delay and the minimum propagation delay can be reduced (the mode dispersion can be reduced) by causing light to propagate with 7th to 10th eigenmodes, as compared with a case where light propagates with the first to tenth modes. In other words, the mode dispersion can be reduced by limiting the oscillation of the lower modes of the 1st to 6th orders and by promoting the oscillation of higher modes. The difference between the maximum propagation delay and the minimum propagation delay can also be reduced (the mode dispersion can be reduced) for wavelengths longer than 850 nm by causing light to propagate in the same manner, as compared with a case where light propagates with the first to tenth modes.
US2007/0217472 discloses a VCSEL having a recess in the center of a distributed Bragg reflecting mirror (DBR) on the light emitting side. The recess overlaps the active layer. The central region of the DBR mirror has a smaller number of stacked mirror layers so that it has a lower reflectance ratio than the peripheral region. Due to such a structure, higher modes having high electric field intensity in the peripheral region oscillate more easily than lower modes having high electric field intensity in the central region.
However, the recessed structure of the DBR mirror on the light emitting side allows laser light to be emitted from the VCSEL with a large emitting angle according to the principle of a lens (concave lens), causes the light to be diffused and increases a loss in coupling with an optical fiber.
An object of the present invention is to provide a VCSEL that limits both the oscillation of lower modes and the angle at which laser light is emitted.